Diphosphanylketenimines: New Reagents for the Synthesis of Unique Phosphorus Heterocycles

Abstract

Two consecutive [3+2] cycloaddition reactions of the diphosphanylketenimine (PPh2)2CCNPh (3), involving the phosphanyl groups, with two equivalents of the electron‐poor alkynes dimethyl acetylenedicarboxylate or methyl acetylenecarboxylate give rise to the formation of the bicyclic 1λ5,3λ5‐diphospholes 5 a,b, which contain a phosphorane unit with five carbon substituents attached to the phosphorus center. Compound 3 undergoes cyclodimerization by crystallization, affording the unsymmetrical dimer 6, which is converted back to 3 by heating in toluene. Compound 6 can be oxidized stepwise on the three trivalent phosphorus atoms by treatment with H2O2 affording 7, 9, and the transient species 10, which are transformed into their corresponding ketenimine monomers either spontaneously (10) or by heating in toluene (7, 9). In this way, the compound (OPPh2)(PPh2)CCNPh (8) is quantitatively obtained. Compound 8 readily reacts with the alkynes MeO2CCCCO2Me and MeO2CCCH, and with phenyl isocyanate and ethyl isothiocyanate through regiospecific [3+2] cycloaddition processes furnishing several λ5‐phosphole and λ5‐azaphosphole derivatives. Finally, the reaction of 8 with N‐methylpropargylamine yields the new 2,3‐dihydro‐1,4‐λ5‐azaphosphinine 15 through a cycloaddition process involving two functional groups from each molecule.

The Continuing Development of the Chemistry of Phospholes

“…Phosphorus can also be placed in the quaternary state by alkylation on sulfur of phosphole sulfides, as seen in the formation of 243 and 244. Ylides based on phospholes are also known, and an unusual example has recently been presented [90]. Here a [3+2] cycloaddition of diphosphanylketenimine 248 and acetylenic esters has given a phosphole ylide structure in products 249 ( 31.6, 29.20, J=34 Hz) and 250 ( 28.40, J=38 z) .…”

“…Examples of this structure are rare, but a reaction related to Equation 71 has given a novel bicyclic structure with this feature, seen in 251 ( 8.97 ylidic P, -81.40, J=71 Hz) and 252 ( -0.50, -84.10, J=85 Mg) [90]. The far upfield 31 P NMR signals are convincing proof of 5-coordinate P; the X-ray diffraction spectrum of the red 251 confirmed the novel structure.…”

Imidazoline‐Functionalized Diphosphines: Models for N‐Heterocyclic Carbene–Diphosphinocarbene Coupling

“…This complex is formed in a cycloaddition process involving the ketenimine fragment of the ligand. This result is in sharp contrast to the reaction of free diphosphinoketenimines with propargylamines, which affords phosphinine derivatives due to the additional involvement of a phosphorus atom in the cycloaddition process 7. As proposed in Scheme , a likely mechanism for the formation of 2 a implies, in a first step, nucleophilic attack of the amine to the ketenimine to afford an enediamine derivative.…”

Phosphines and Related Tervalent Phosphorus Systems

“…465 Two consecutive [3þ2]-cycloadditions of the diphosphinoketenimine (174) with acetylenic esters give rise to the bicyclic 1l 5 ,3l 5 -diphospholes (175). 466 The reactivity of tertiary phosphines towards alkenes bearing electron-withdrawing groups has also continued to attract attention. Triphenylphosphine in refluxing methanol reduces maleimides to succinimides in good yield.…”